Critical Angle Calculator

About this calculator

This calculator finds the critical angle for total internal reflection using Snell's law: θc = arcsin(n2 / n1), where n1 is the refractive index of the denser medium light is traveling through and n2 is the refractive index of the less dense medium beyond the boundary. Beyond this angle of incidence, light no longer refracts out — it reflects entirely back into the denser medium.

Optical engineers, fiber-optics technicians, gemologists, and physics students use it to understand why light stays trapped inside an optical fiber core, to judge a gemstone's brilliance from its critical angle and cut, and to design prisms and light pipes. Enter the two refractive indices to get the critical angle.